The invention relates to a method for measuring a load impedance of a load circuit connected to an SLIC circuit.
Analog terminal devices, for example analog telephone terminal devices can be connected to a digital communications network via a two-wire telephone line, an SLIC circuit and a Codec circuit.
The connecting line and the analog terminal device have a complex load impedance (ZL). A possible cable break or a possible line break and the type of connecting line via which the analog terminal device is connected to the SLIC circuit can be detected by measuring the load impedance ZL of the load circuit.
The object of the present invention is therefore to provide a method with which the load impedance of a load circuit connected to an SLIC circuit can easily be measured.
This object is achieved according to the invention by means of a method having the features disclosed in claim 1.
The invention provides a method for measuring a load impedance of a load circuit which is connected to an SLIC circuit of an analog terminal connection of a terminal device having the following steps, specifically:
a digital toll signal is generated by means of a Codec circuit connected to the SLIC circuit, said toll signal being converted into an analog toll signal;
the analog toll signal is output by the SLIC circuit to the load circuit;
an analog current which is brought about at the terminal connection of the terminal device via the analog toll signal and is converted into a corresponding voltage is sensed;
the digital toll signal is filtered by means of an adaptive filter which is provided in the Codec circuit and has adjustable filter coefficients for generating a filtered digital comparison signal which is converted into an analog comparison voltage;
the filter coefficients of the adaptive filter are adjusted until the analog comparison voltage and the analog voltage, brought about by the analog toll signal, at the terminal connection of the terminal device are the same;
the complex load impedance of the load circuit is calculated as a function of the set filter coefficients.
The digital toll signal is preferably multiplied in the adaptive filter by a first filter coefficient in order to generate a weighted digital toll signal.
The digital toll signal is preferably phase-shifted in the adaptive filter by means of a phase shifter and is multiplied by a second filter coefficient in order to generate a second weighted digital toll signal.
The first weighted digital toll signal and the second weighted digital toll signal are preferably summed to form the digital comparison signal.
The analog comparison voltage is preferably subtracted from the analog voltage which is brought about at the terminal connection of the terminal device by the analog toll signal in order to generate an analog difference signal.
The analog difference signal is preferably converted into a digital difference signal.
The digital difference signal is preferably multiplied by the digital toll signal and is integrated in order to generate the first filter coefficient of the adaptive filter.
The digital difference signal is preferably multiplied by the phase-shifted digital toll signal and is integrated in order to generate the second filter coefficient of the adaptive filter.
The digital toll signal is phase-delayed by preferably 90xc2x0 by the phase shifter.
The analog toll signal is preferably a sinusoidal signal.
The sinusoidal toll signal preferably has a frequency of 12 or 16 kHz.
The analog toll signal is preferably amplified by the SLIC circuit.
The complex load impedance of the load circuit is preferably calculated as a function of the first filter coefficient and the second filter coefficient of the adaptive filter and as a function of a constant complex factor.
A preferred embodiment of the method according to the invention for measuring a load impedance of an analog load circuit which is connected to an SLIC circuit is described below with reference to the appended figures in order to explain features which are essential to the invention.